1. Field of the Invention
The present invention relates to a motor drive circuit and a motor drive system using the same, and more particularly relates to a motor drive circuit formed in an IC which prevents a transistor in the output stage thereof from being damaged when the motor is in a short-braking operation.
2. Background Art
FIG. 6 is a circuit diagram showing an example of conventional motor drive circuits. FIG. 7 is a view showing a structure of transistors Q1, Q2 and Q3 on a semiconductor chip when the motor drive circuit in FIG. 6 is formed in an IC.
The transistor Q1 is a PNP type transistor which drives the transistor Q2 at the output stage, and the emitter of the transistor Q1 is connected to a power source line Vcc, the collector thereof is connected to the base of the transistor Q2 and the base thereof is adapted to receive a driving signal. The transistor Q2 is an NPN type transistor, the collector thereof is connected to the power source line Vcc, and the emitter thereof is connected to a output terminal 2 and the collector of the transistor Q3. The transistor Q3 is an NPN type transistor which turns on during a short braking operation, the emitter thereof is connected to a GND line to ground the same and the base thereof is adapted to receive a control signal for a short braking. To the output terminal 2 a three phase motor such as for a fan is connected.
A motor driving circuit for only one selected phase is explained hereinbelow, because those for the other two phases are identical to that explained. For driving the motor 1 the transistor Q1 at a drive stage for the first time is turned on and in response thereto the transistor Q2 at the output stage is turned on to allow a current flow to the motor 1 from the power source line Vcc via transistor Q2. As a result the motor is driven.
In order to stop the motor 1, the motor 1 has to enter into a short braking condition in which the transistor Q1 is turned off and the transistor Q3 at the output stage is turned on, and thereby the output terminal 2 is conductively connected to the GND terminal. As a result, the current flowing through the motor 1 flows toward the GND line. Further, by turning off the transistor Q1, the transistor Q2 is turned off and the current from the power source line Vcc to the motor 1 is interrupted.
During the short braking operation, a counter electromotive force is generated in the motor and the potential of the output terminal 2 is rendered negative. At this moment, a parasitic transistor Q4 indicated by dotted line in FIG. 6 turns on. As a result, the base potential of the transistor Q1 lowers and the transistor Q1 is turned on. Subsequently, the transistor Q2 also turns on. The parasitic transistor Q4 is formed by the base 6 of the transistor Q1 a its collector, a P type substrate 3 as its base and the collector 7 of the transistor Q3 as its emitter. When the transistor Q2 is turned on, a large current flows into the transistor Q2. Therefore when such a motor drive circuit is formed in an IC, a large current flows into the transistor Q2 during a short braking operation, which possibly damages the transistor Q2.